In the past, numerous designs have been proposed for multifocal contact lenses. One of the desires in the art has been to provide a contact lens solution that more satisfactorily addresses the condition of presbyopia, caused by the loss of elasticity or flexibility of the eye's crystalline lens. Presbyopia limits the ability of the natural lens to focus upon near objects. For the patient needing the combination of near vision correction and far vision correction, conventional solutions have included bifocal glasses, the combination of contact lenses, providing distance correction only, with reading glasses, and the provision of a pair of contact lenses each having a different prescription, i.e., a first lens providing one eye with distance correction and a second lens providing the other eye with near vision correction, sometimes referred to as monovision.
Many designs of multifocal contact lens products have become available, but various limitations have resulted in unsatisfactory visual acuity under certain conditions. One such type of multifocal contact lens geometry that has become common is based upon dividing the lens region into concentric optical zones, i.e., a most central optic zone surrounded by one or more distinct and concentrically oriented annular outer zones. When the lens is limited to two constant power zones it has been possible to provide good visual acuity at two distinct distances from the eye, but because the dimensions of the pupil vary from person to person and as a function of illumination, the effective optical area of the outer zone can vary significantly. With this variation the lens performance for near vision or distance vision may be compromised.
To reduce such dependence on pupil size and resulting imbalance of light throughput between near and distance zones of the lens, it has been proposed that additional concentric zones be incorporated in the contact lens in an alternating pattern. See U.S. Pat. No. 4,704,016. While this approach addresses the need for a more balanced throughput at various levels of illumination, it is known that the inclusion of numerous discrete zones can result in creation of diffractive edge effects at the junctions between such zones.
It is also known to provide an aspheric zone between the distance and near vision zones. Such may create a continuous gradient of optical power over a selected range. However, this approach has also been limited by the introduction of diffractive edge effects as well as the influence of pupil size on optical throughput and visual acuity. Moreover, to the extent such systems have been implemented, they have not provided consistent results.
With such limitations and deficiencies there is a need for a contact lens design which provides more acceptable performance for the combination of distance vision, intermediate vision and near vision.